Repeated exposure to sham testing procedures reduces reflex withdrawal and hot-plate latencies: attenuation of tonic descending inhibition?

Five days' repeated exposure of experimentally naive rats to the experimental environment and to sham nociceptive testing procedures ('habituation') reduced the latency for reflex withdrawal of the hindpaw from hot water (49 degrees C) by 43%, to that of spinalised habituated or novice animals. Hot-plate (50 degrees C) paw lick latencies were reduced equally (40%) by habituation or parachlorophenylalanine, and were increased 32% by D,L-5-hydroxytryptophan. Neither drug affected hot-plate latencies of habituated animals. Naloxone had no effect on flexor withdrawal or hot-plate latencies in either novice or habituated animals. These results suggest that habituation substantially attenuates tonic serotonergic inhibition of spinal nociceptive transmission.